Petroleum fuels have long been predominantly used in industry and transport world-over. However, these fuels have a limitation of availability, and they also produce high levels of emissions especially carbon oxides viz carbon dioxide (CO2), carbon monoxide (CO). Hydrogen (H2) has been suggested to be a good alternative to replace conventional petroleum fuels. H2 utility as a substitution to fossil fuels has attracted much attention in the last two decades because of its successful demonstration in space technologies and fuel cells, although it necessitates the use of a complex and costly manufacturing process. Conventional methods for production of hydrogen are methane steam reforming, methane partial oxidation or coal gasification routes. However, in addition to production of hydrogen, these methods also produce considerable amounts of carbon oxides (COx) like carbon dioxide, carbon monoxide and their separation is costly and detrimental to the electrodes used in fuel cells. Thus, hydrogen obtained through conventional methods has to be purified to render it free from COx for fuel cell application and other applications. The conventional methods used for hydrogen production involve multistep operations and are not commercially viable.
The production of COx free hydrogen could be advantageous in terms of environmental and economic aspects. The routes proposed for the production of clean hydrogen are ammonia (NH3) decomposition, hydrogen splitting and catalytic decomposition of methane (CDM) or lower hydrocarbons. The later method is inexpensive as compared to the former methods for production of COx free hydrogen. The catalytic decomposition of methane can be presented as following.CH4→C+2H2 ΔH=17.8 kcal/mole
CDM to pure hydrogen has several advantages as compared to conventional hydrogen production methods. CDM yields pure hydrogen at less severe conditions. Further, the process results in simultaneous production of high value nano-carbons along with hydrogen. This makes the above process as an important alternative process to produce COx free hydrogen. On the other hand, an important disadvantage of the process is that it produces carbon deposits on the catalyst that result in the catalyst deactivation. The regeneration of the deactivated catalyst is done by the combustion of carbon deposits which leads to generation of carbon dioxide. CDM on Ni, Fe and Co based catalysts on inert support materials to give hydrogen is reported. Ni based catalysts are more active and stable than other transition metals and results in higher hydrogen and carbon yield for methane decomposition (Zhang et al. Catal. Lett. 2004, 7).
Methane is decomposed to yield COx free hydrogen and carbon on catalyst based on NixMgyO (where x and y represent the mole content of Ni and Mg). Addition of Cu as a promoter to the catalyst composition enhances both methane decomposition and solid carbon yield and increases the catalyst life time up to 19 h (US2005/0063900). Methane decomposition in the presence of a catalyst based on. Fe and Ni to give hydrogen enriched fuel and carbon nanotubes have been carried out by using microwave irradiation. In this process, both the catalyst and methane were exposed to microwave irradiation at a selected microwave power (US2008/0210908). Catalysts synthesized by admixing Fe salt and in combination with Ni, Pd, and Mo have been used for decomposition of light hydrocarbons to hydrogen and carbon nanomaterial. The binary metal salts having at least Fe as one metal and in combination with Ni, Pd, Mo were found to be active for the production of hydrogen at the temperature range of 500 to 1000° C. (U.S. Pat. No. 6,875,417). Multi walled, size controlled carbon nanotubes were produced by the decomposition of carbon containing compounds over supported transitional metal based catalysts. The typical support materials used for the process were SiO2, SiO2/Al2O3, aerogel Al2O3 and MgO (U.S. Pat. No. 7,214,360).
Unsupported nano-sized nickel oxide particles have been utilized for the production COx free hydrogen by methane decomposition especially at low temperatures, between 300 to 500° C. It has been observed that the catalyst performance is strongly dependent on the particle size of the catalyst (US2009/0140215). Production of carbon nanotubes can be achieved with the dimension of 3-150 nm having the aspect ratio of more than 100 by the decomposition of hydrocarbons over solid catalyst containing Co, and Mn on an inert support (US2009/0140215). Lower hydrocarbon in the presence of low concentration of oxidizing/reducing gas or moisture can be subjected to decomposition in the presence of catalyst to yield functional carbon nanomaterials and hydrogen. The amorphous carbon produced on the catalyst is removed (U.S. Pat. No. 7,767,182). Silica supported Ni catalyst have been used for catalytic methane decomposition at low reaction temperatures (550° C.) and these catalysts have produced long cylindrical hollow carbon filaments (Zhang et al. Appl. Catal. A: Gen 1998, 161).
One major drawback of the processes for the conversion of methane to give carbon oxides free hydrogen and carbon nanotubes is the rapid deactivation of the active catalyst. Further, the active catalyst deactivates at a rapid rate due to higher amorphous carbon deposition. The conversion of methane is in the range of 50-60% with low carbon yield. Although considerable research has been done on the activity of different catalysts for methane decomposition reaction, there is no effective catalyst available that operates with lesser deactivation for a longer time. While the concepts for catalytic decomposition of methane for production of hydrogen have been shown in the state of the art methods, there still exists a need to develop novel catalytic compositions for decomposition of lighter hydrocarbons to yield carbon nanotubes and COx free hydrogen